1. Field of the Invention
The present invention relates to high pretilt and photo-stable liquid crystal displays. More particularly, it relates to a method for meeting the ever more stringent requirements for liquid crystal devices used in projection applications.
1. Background Art
The alignment of liquid crystal display degrades under intense photo irradiation in projection displays. The problem becomes worse when the size of the display is smaller and the brightness of the display is higher. The photo-stability of the liquid crystal display has becoming a major barrier preventing projection liquid crystal displays from entering high-end display market such as HDTV and digital cinema. Recent improvements of polyimide, which is the industrial standard of liquid crystal displays, has extended the lifetime of the liquid crystal from 5-8 MJ/cm2 to about 12-15 MJ/cm2. However, it is still not enough. A test conducted in early 2003 revealed that the LCD projectors failed in 3-4 weeks with 24/7 operation. Therefore, the quest for a photo-stable alignment technology is very important for the LCD projection industry. In addition to photostability, fast response time is another requirement for projection displays to be used for video applications. Higher pretilt angle, the angle between liquid crystal molecules and substrates when no electrical field is applied to the display, is one of the most common approaches used to enhance response time. Usually, the alignment with high pretilt is not stable. Therefore, a photo-stable and high pretilt homogeneous alignment remains a challenge for projection liquid crystal displays.
In U.S. Pat. No. 6,195,146, a liquid crystal display cell includes a first substrate; a second substrate; a liquid crystal layer of liquid crystal molecules disposed between the first substrate and the second substrate; a first alignment layer disposed between the first substrate and the liquid crystal layer; and a second alignment layer disposed between the second substrate and the liquid crystal layer. A surface of at least one alignment layer is bombarded by a particle beam of an adjustable energy using a voltage less than 200 volts so that the liquid crystal molecules proximate the surface are induced to a predetermined pretilt angle.
U.S. Pat. No. 6,124,914 teaches a method of generating a patterned alignment direction on an alignment surface for a liquid crystal display cell. The cell is formed by forming a first alignment direction on an alignment surface, such as a polyimide surface of the cell. A second alignment direction is formed on the alignment surface. The first and second alignment directions are formed by a variety of sequences of treatments. An example of a first sequence is a first step of rubbing the alignment surface and thereafter a second step of selectively exposing (preferably using a mask) the alignment surface to a treatment selected from exposure to electromagnetic radiation and exposure to a particle beam. Another example of a sequence is a first step of exposing the alignment surface to a treatment selected from electromagnetic radiation and a particle beam and thereafter selectively exposing (preferably through a mask) the alignment surface to another particle beam.
In U.S. Pat. No. 6,346,975, a liquid crystal display device has first and second substrates, a first electrode layer overlying one surface of the first substrate, and a second electrode layer overlying one surface of the second substrate. A first alignment layer overlies the first electrode layer, and a second alignment layer overlies the second electrode layer, and a liquid crystal material is disposed between the alignment layers. In one preferred embodiment, each alignment layers is a polyimide layer. This patent also teaches a method for manufacturing a liquid crystal display device. According to the method, first and second substrates are provided, a first electrode layer is deposited over the first substrate, and a second electrode layer is deposited over the second substrate. A first alignment layer is deposited over the first electrode layer, and a second alignment layer is deposited over the second electrode layer. The first and second substrates are arranged so that the alignment layers face one another and a space is formed between the substrates. A liquid crystal material is disposed in the space between the first and second substrates.
The general teachings of these patents is incorporated herein by reference.